The present invention relates to an exhaust gas purifying system for an automotive vehicle having an automatic transmission. Under predetermined conditions the return of the throttle valve of the carburetor to the idling position when the vehicle is decelerated is prevented thereby decreasing the unburned compounds in the exhaust gases. Under other predetermined conditions an effective engine brake is fully maintained.
In general, when automotive vehicles are decelerated, the volumetric efficiency of an internal combustion engine is increased so as to decrease the unburned compounds in the exhaust gases. For this purpose, there have been devised and demonstrated various methods. In one method, a throttle valve in a carburetor is opened to some extent in response to the rise of negative pressure in the intake manifold in excess of a predetermined magnitude or the fuel-air mixture is injected downstream of the throttle valve. In another method, stop means is provided to return the throttle valve to the idling position. However, the device for increasing the volumetric efficiency when the vehicle is decelerated, thereby decreasing the unburned compounds in the exhaust gases has a distinct defect in that the efficiency of the engine braking is not satisfactory when the vehicle is decelerated. In an automotive vehicle with an automatic transmission, the shift lever is shifted to the low or second range in order to produce the engine brake because when the shift lever is in the drive range or automatic shift range, the automatic transmission is automatically shifted up from the second range into the third range as the speed of the vehicle is increased. In an automotive vehicle, that is an automative vehicle with an automatic transmission incorporating the exhaust gas purifying system of the type described, the volumetric efficiency is also increased in the low or second range so that the efficiency of the engine braking is considerably reduced. That is, shifting down into the low or second range does fail to produce efficient engine braking. As a result, even when accelerator pedal is released, engine braking is not produced as expected so that the foot brake must be frequently applied when the vehicle is going down a long downhill. Therefore, there arises the problem of the overheat of the brake, a safety hazard.